The present invention relates to an ultrasound imaging system and method thereof, more particularly, to a pulse-compression based on ultrasound imaging system and method that generates a harmonic image with an enhanced signal-to-noise ratio (SNR) by effectively removing fundamental frequency components through pulse-compression using weighted chirp signals.
The ultrasound imaging system is widely used in the medical field for displaying an image of a target object, such as a human body. Ultrasound signals are transmitted to the target object and then reflected from the target object, thereby forming the ultrasound image.
For transmission of the ultrasound signals, the ultrasound imaging system generally includes a transducer array, which includes a plurality of transducers and a pulser for driving each transducer. Each transducer generates ultrasound signals in response to the pulse applied from the pulser. During transmission of the ultrasound signal, the timing of generating the ultrasound signal at each transducer is controlled, thereby transmit-focusing the ultrasound signals at a predetermined position within the target object. In other words, the pulser pulses the respective transducers with different time delays so that the ultrasound signals reach a desired position within the target object at the same time.
The ultrasound signals reflected from the target object are received by the transducer array. The time for the reflected signals to reach the respective transducers is different depending on the distance between the transducers and the target object. Therefore, in order to compensate for the time difference among the transducers, a beamformer applies time delays, with respect to the reflected signals, which are received by the respective transducers, and generates receive-focused signals.
The power of the received signals is remarkably lowered when the ultrasound signal passes through a highly dense medium, such as a human body. As a result, when the target object is located deep in the body, the desired information is difficult to obtain with the above-mentioned ultrasound imaging apparatus. Most of the ultrasound imaging apparatuses currently being used generate ultrasound signals by applying a pulse of short duration to the transducers. Increasing the peak voltage of the pulse may solve problems due to the attenuation of the ultrasound signals. However, there is a certain limit to increasing the peak voltage of the pulse since this may affect the internal organs of the human body.
Ultrasound signals are distorted by various nonlinear characteristics of the propagation medium, which give rise to phenomena such as diffraction and attenuation. These nonlinear characteristics distort a transmitted ultrasound signal to generate harmonic frequency components of the signal frequency. The imaging technique using harmonic frequency components is called xe2x80x9cultrasound harmonic imaging techniquexe2x80x9d. In general, harmonic frequency components are generated at integer multiples of the fundamental frequency components. The nonlinear propagation of ultrasound waves can be modeled via the Khokhov-Zabolotskaya-Kuznetsov (KZK) equation, which can be solved by a finite difference approximate scheme.
The ultrasound harmonic imaging technique is geared to forming an ultrasound image by using a second harmonic frequency component that is produced by the nonlinear characteristics of the medium in response to transmission of a short pulse. Thus, forming an ultrasound image by using harmonic frequency components has proven to produce an ultrasound image having an improved resolution, SNR and contrast as compared to an ultrasound imaging method using only a fundamental frequency component. Therefore, the ultrasound imaging method using harmonic frequency components forms an ultrasound image of better image quality than the method using a fundamental frequency component.
Since the harmonic frequency components are generated in proportion to the intensity of the sound pressure, and the magnitude of the harmonic frequency components are much lower than the fundamental frequency component, an ultrasound signal with sufficient sound pressure should necessarily be used as a transmission signal. However, conventional ultrasound harmonic imaging techniques employ a short pulse signal so there is a limit to increasing the sound pressure of the ultrasound transmission signal and the SNR. If the sound pressure of the ultrasound signals increases to above predetermined threshold value, then a saturation phenomenon arises wherein harmonic frequency components are not increased any more. Accordingly, conventional ultrasound imaging methods cannot improve the SNR of a harmonic image, typically, a second harmonic image beyond a predetermined level by increasing the sound pressure of the ultrasound transmission signal.
In order to form an ultrasound image of high quality with an ultrasound harmonic imaging technique, the fundamental frequency component should be removed and harmonic frequency components extracted from the received ultrasound signal. For such purposes, general filtering method, such as those using a band-pass-filter (BPF) or high-pass-filter (HPF) is commonly used.
However, if the frequency band of the fundamental frequency components overlap with the harmonic frequency components, then the harmonic frequency components may undesirably be filtered out in proportion to the overlapping bandwidth, or the fundamental frequency components not completely removed, thereby deteriorating the SNR and resolution of the ultrasound image. To circumvent the drawbacks associated with the filtering method, the pulse inversion method may alternatively be used which revealed to be more effective in eliminating a fundamental frequency component than the filtering method.
The pulse inversion method transmits two ultrasound signals, a positive polarity pulse and a negative polarity pulse, that have a phase difference of 180xc2x0from each other along every scan line, and then adds the two received ultrasound signals, thereby effectively removing the fundamental frequency component.
However, the pulse inversion method also has problems from the system perspective since two transmit-receive steps are required to form one scan line, and therefore, the frame rate is cut in half when the pulse inversion method is applied for all the scan lines.
To avoid the limitation on transmission sound pressure and increase the ultrasound penetration distance, a conventional fundamental frequency imaging method may employ pulse compression. An ultrasound imaging system employing a pulse compression method uses a coded long pulse of a long duration time instead of a conventional short pulse. In such an ultrasound imaging system influences the system a particular pulse being used influences the system performance to a great degree. That is, the ultrasound image quality is affected by how well matched the frequency band of the used signal is to the limited band characteristic of the transducer array.
System performance is further affected by the specific configuration of the correlator or pulse-compressor in an ultrasound receiver. Note that where coded long pulses are used, the correlator or pulse-compressor is used to give the same effects as the short pulse.
The conventional ultrasound imaging system employing pulse compression includes a single correlator matched to the fundamental frequency component in the ultrasound receiver so that an ultrasound image is formed with only the fundamental frequency component. Accordingly, a harmonic image cannot be formed with the conventional ultrasound imaging system. Thus, the quality of a conventional ultrasound image is lowered.
The reason why only a single correlator only matched to the fundamental frequency component has been inevitably used is that the magnitudes of the harmonic frequency components in the received ultrasound signal are very low relative to that of the fundamental frequency component so that it is not feasible to extract the harmonic frequency components from the received ultrasound signal. The additional reason that oars using multiple correlators in the ultrasound receiver lies in that an appropriate signal is not yet available which matches well to the limited band characteristics of the transducer array. Even though spread spectrum signal, such as a chirp signal, may reasonably be matched to the limited band characteristics of the transducer array, such a general chirp signal, if subjected to a correlator results in a peak sidelobe whose gain is that of lower than a mainlobe by approximately xe2x88x9213 dB. However, for medical ultrasound imaging applications, the output signal of a pulse-compressor should have sidelobes of xe2x88x9250 dB or less. So, the general chirp signal is not well matched to the specifications of medical ultrasound imaging systems.
It is, therefore, an objective of the present invention is to provide an ultrasound imaging system and method to generate a harmonic image with an enhanced SNR(signal-to-noise ratio) by effectively removing a fundamental frequency component through pulse-compression using weighted chirp signals.
Another objective of the present invention is to provide an ultrasound imaging system and method that increase the SNR of a harmonic image by extending the length of a weighted chirp signal using a weighted chirp signal, which is to be transmitted in a form of ultrasound signals.
Still another objective of the present invention is to provide an ultrasound imaging system and method that selectively pulse-compresses only harmonic frequency components by transmitting ultrasound signals converted from weighted chirp signals and allowing RF samples received at each transducer to pass through a correlator matched with harmonic frequency components of the transmitted ultrasound signals.
According to one aspect of the present invention, an ultrasound imaging system includes: a transducer array for converting weighted chirp signals to ultrasound signals, and transmitting the ultrasound signals to a target object; a receiver for receiving signals reflected from the target object; a pulse-compressor for pulse-compressing harmonic frequency components of the ultrasound signals in the reflected signals; and means for producing receive-focusing the pulse-compressed signals.
The pulse-compressor further includes: a selector for selecting the harmonic frequency components in the reflected signals; and a correlator for pulse-compressing the selected harmonic frequency components.
According to another aspect of the present invention, an ultrasound imaging method includes the steps of: converting weighted chirp signals to ultrasound signals; transmitting the ultrasound signals to a target object; receiving signals reflected from the target object; pulse-compressing harmonic frequency components of the ultrasound signals in the reflected signals; and receive-focusing the pulse-compressed signals.
The pulse-compressing step further includes selecting the harmonic frequency components in the reflected signals; and pulse-compressing the selected harmonic frequency components selected by the selector.
According to still another aspect of the present invention, an ultrasound imaging system includes: a transducer array for converting weighted chirp signals to ultrasound signals, and transmitting the ultrasound signals to a target object; a receiver for receiving signals reflected from the target object; a pulse-compressor for selectively pulse-compressing fundamental frequency components or harmonic frequency components of the ultrasound signals in the reflected signals; and means for receive-focusing the pulse-compressed signals.
The pulse-compressor further includes: a first correlator for pulse-compressing the fundamental frequency components; a second correlator for pulse-compressing the harmonic frequency components; and a mode selector for selecting the fundamental frequency components or the harmonic frequency components in the reflected signals and for enabling the selected frequency components to be pulse-compressed via one of the first and second correlators.
According to still another aspect of the present invention, an ultrasound imaging method includes the steps of: converting weighted chirp signals to ultrasound signals, and transmitting the ultrasound signals to a target object; receiving signals reflected from the target object; pulse-compressing fundamental frequency components or harmonic frequency components of the ultrasound signals in the reflected signals; and receive-focusing the pulse-compressed signals.
The pulse-compressing step further includes the steps of: selecting the fundamental frequency components or the harmonic frequency components in the reflected signals; and pulse-compressing the selected frequency components.